In some practical imaging applications, it is desired to scan a sample by partitioning the sample into an array of fields of view (FoVs) and scanning the array of FoVs in a raster-scanning manner. Before an image appeared on one FoV is captured, focusing is performed in order to bring this image in-focused. That is, focusing is individually applied to each of the FoVs. One example of such practical imaging applications is related to clinical pathology. A pathology slide with tissue or cells for scanning and examination typically has an area of 5 cm by 2 cm while a FoV that can be imaged by a microscope is around 1 mm in diameter. Since high-resolution images are required to reveal fine details of tissue and cells high-magnifying objective lenses are used. These lenses have very shallow depths of view so that a slight sample-surface roughness or stage tilt can lead to image blurring. Thus, focusing is required for each FoV. Another example of such practical imaging applications is related to imaging semiconductor dice on a wafer with a high degree of magnification in order to reveal small features on each semiconductor die.
EP2390706 and U.S. Pat. No. 7,232,980 suggest an image scanning system using a dedicated autofocus imaging sensor, separate from a primary imaging sensor, for achieving focusing of the FoVs. The presence of the separate, dedicated autofocus imaging sensor is contributory to enable the image scanning system to achieve high-speed scanning. For the purpose of determining how much optical adjustment for achieving focusing, the dedicated autofocus imaging sensor is tilted so that this autofocus imaging sensor is not positioned perpendicular to an optical axis of the image scanning system. U.S. Pat. No. 7,232,980 further suggests an alternative implementation of the image scanning system in which the dedicated autofocus imaging sensor can be positioned perpendicular to the optical axis provided that an optical path-length changing member is put in front of the autofocus imaging sensor. The optical path-length changing member effectively rotates the focus plane of an image that passes therethrough, thereby introducing an effect substantially similar to an effect of tilting the autofocus imaging sensor for achieving focusing.
In the disclosure of EP2390706 and U.S. Pat. No. 7,232,980, however, the autofocus imaging sensor can only be physically tilted in one direction or the optical path-length changing member can rotate the focus plane in one direction only. To achieve focusing with improved accuracy or greater reliability, it is advantageous if multiple directions are involved in performing focusing. Providing multiple directions is achievable by using multiple autofocus imaging sensors or using multiple optical devices each of which is a cascade of an optical path-length changing member and an autofocus imaging sensor. However, the implementation cost is inevitably increased.
It is desirable if a single autofocus imaging sensor is used while multiple directions are provided. Since high-speed scanning of FoVs is desirable for many imaging applications, predictive focusing can be advantageously employed in high-speed image scanning systems to ensure that high-sharpness images can be obtained in such high-speed systems. There is a need in the art for an apparatus that employs a single autofocus imaging sensor but provides multiple directions for achieving predictive focusing in an image scanning system. An example of the image scanning system is a microscope system.